Information playback systems frequently utilize a stylus for reading signals from the surface of an information record, typically a plastic disc that contains stored video and audio information. In some systems, the information record has a fine spiral groove to guide the tip of a stylus that contains a thin electrode. In these systems, the stylus tip is made of a material having sufficient hardness to withstand the abrasion caused from tracking the groove. Materials which possess such hardness, such as diamond, generally have a crystallographic structure which presents surfaces exhibiting different qualities depending upon which crystallographic plane the surfaces are oriented along. The video disc stylus utilized in the CED (capacitance electronic disc) system is tapered to form the prow of the tip, and is also lapped to form a keel having a V-shaped shoe for its bottom portion. The keel-shaped tip has a shoe length of about 3 to 5 .mu.m and a thickness of about 2 .mu.m. Making a long-shanked stylus entirely from the same material may become expensive, particularly when the tip material, for example, diamond, exceeds the cost of other suitable materials from which the shank can be made.
In order to reduce manufacturing costs, the shank of the stylus may be made from a different material which is less expensive than the crystallographic tip material. For example, a small diamond stone may be mounted at the end of a relatively long metallic shank, such as a cylindrical titanium rod. The diamond stone utilized may be a synthetic diamond stone which is less expensive to obtain than a natural diamond stone. The synthetic diamond stone has a plurality of facets oriented along the {100} family of planes and a plurality of facets oriented along the {111} family of planes. The diamond stone is typically mounted in a concave-shaped cavity at the end of the metallic shank by any means capable of holding the stone therein, such as utilizing a braze, setting the stone in a pocket of adhesive epoxy, a braze, or the like.
In fabricating the keel-shaped tip from the shank-mounted diamond stone, the end at which the stone is mounted is first coned in order to form a conical diamond tip and also to remove a portion of the surrounding shank metal so that it will not interfere with subsequent stylus processing. A novel apparatus for performing this coning is described in U.S. Pat. No. 4,417,423 issued Nov. 29, 1983 to E. F. Cave and J. J. Cowden entitled "Stylus Coning Fixture." The coning operation disclosed in that patent produces a prismatic cone which exhibits four-fold symmetry due to the anisotropic hardness of the diamond crystalline structure. The crystallographic directions are automatically revealed thereby providing an alignment means for orienting the stylus during subsequent processing. See also U.S. Pat. No. 4,403,453 issued Sept. 13, 1983 to E. F. Cave and J. J. Cowden entitled "Stylus Coning Fixture" for a stylus tip positioning apparatus.
Following the coning step on the tip, an electrode facet is lapped along one of the sides of the prismatic cone. After depositing a conductive electrode onto this electrode facet, two intersecting prow facets are lapped adjacent the electrode facet. In the manufacture of a plurality of substantially uniform styli, it is desirable that the lapping of the electrode and prow facets be carefully controlled, especially since the rate of machining a particular facet, and consequently the time required for lapping that facet, may vary from one stylus to the next.
See U.S. Pat. Nos. 3,902,283 issued Sept. 2, 1975 and 4,106,240 issued Aug. 15, 1978 for descriptions of apparatus that facet gems, but not suitable for faceting a plurality of stylus tips rapidly and without damaging the tips during the set down operation.
To control precisely the lapping of the styli and thereby achieve styli of substantially uniform shape, the present invention provides a novel apparatus for softly setting the stylus tip down on a rotating scaife and for uniformly lapping a facet at the tip of a stylus without damaging the stylus tip.